Mobile Communications
Chapter 3 : Media Access
Motivation
 SDMA, FDMA, TDMA
 Aloha
 Reservation schemes

Collision avoidance, MACA
 Polling
 CDMA
 SAMA
 Comparison

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.1
Motivation
Can we apply media access methods from fixed networks?
Example CSMA/CD


Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision
occurs (original method in IEEE 802.3)
Problems in wireless networks

signal strength decreases proportional to the square of the distance
 the sender would apply CS and CD, but the collisions happen at the
receiver
 it might be the case that a sender cannot “hear” the collision, i.e., CD does
not work
 furthermore, CS might not work if, e.g., a terminal is “hidden”
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.2
Motivation - hidden and exposed terminals
Hidden terminals

A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B, A cannot receive the collision (CD fails)
 A is “hidden” for C
Exposed terminals
A
B
C

B sends to A, C wants to send to another terminal (not A or B)
 C has to wait, CS signals a medium in use
 but A is outside the radio range of C, therefore waiting is not
necessary
 C is “exposed” to B
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.3
Motivation - near and far terminals
Terminals A and B send, C receives

signal strength decreases proportional to the square of the distance
 the signal of terminal B therefore drowns out A’s signal
 C cannot receive A
A
B
C
If C for example was an arbiter for sending rights, terminal B would
drown out terminal A already on the physical layer
Also severe problem for CDMA-networks - precise power control
needed!
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.4
Access methods SDMA/FDMA/TDMA
SDMA (Space Division Multiple Access)

segment space into sectors, use directed antennas
 cell structure
FDMA (Frequency Division Multiple Access)

assign a certain frequency to a transmission channel between a sender
and a receiver
 permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping
(FHSS, Frequency Hopping Spread Spectrum)
TDMA (Time Division Multiple Access)

assign the fixed sending frequency to a transmission channel between a
sender and a receiver for a certain amount of time
The multiplexing schemes presented in chapter 2 are now used to control
medium access!
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.5
FDD/FDMA - general scheme, example GSM
f
960 MHz
935.2 MHz
124
200 kHz
1
20 MHz
915 MHz
890.2 MHz
124
1
t
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.6
TDD/TDMA - general scheme, example DECT
417 µs
1 2 3
downlink
11 12 1 2 3
uplink
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
11 12
t
MC SS05
3.7
Aloha/slotted aloha
Mechanism

random, distributed (no central arbiter), time-multiplex
 Slotted Aloha additionally uses time-slots, sending must always
start at slot boundaries
Aloha
collision
sender A
sender B
sender C
t
Slotted Aloha
collision
sender A
sender B
sender C
t
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.8
DAMA - Demand Assigned Multiple Access
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming
Poisson distribution for packet arrival and packet length)
Reservation can increase efficiency to 80%




a sender reserves a future time-slot
sending within this reserved time-slot is possible without collision
reservation also causes higher delays
typical scheme for satellite links
Examples for reservation algorithms:



Explicit Reservation according to Roberts (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.9
Access method DAMA: Explicit Reservation
Explicit Reservation (Reservation Aloha):

two modes:

ALOHA mode for reservation:
competition for small reservation slots, collisions possible
 reserved mode for data transmission within successful reserved slots (no
collisions possible)

it is important for all stations to keep the reservation list consistent at any
point in time and, therefore, all stations have to synchronize from time to
time
collision
Aloha
reserved
Aloha
reserved
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Aloha
reserved
MC SS05
Aloha
3.10
t
Access method DAMA: PRMA
Implicit reservation (PRMA - Packet Reservation MA):

a certain number of slots form a frame, frames are repeated
 stations compete for empty slots according to the slotted aloha
principle
 once a station reserves a slot successfully, this slot is automatically
assigned to this station in all following frames as long as the station
has data to send
 competition for this slots starts again as soon as the slot was empty
in the last frame
reservation
ACDABA-F
1 2 3 4 5 6 7 8
time-slot
ACDABA-F
frame1 A C D A B A
F
AC-ABAF-
frame2 A C
A---BAFD
frame3 A
B A F
ACEEBAFD
frame4 A
B A F D
A B A
frame5 A C E E B A F D
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
collision at
reservation
attempts
t
MC SS05
3.11
Access method DAMA: Reservation-TDMA
Reservation Time Division Multiple Access

every frame consists of N mini-slots and x data-slots
 every station has its own mini-slot and can reserve up to k data-slots
using this mini-slot (i.e. x = N * k).
 other stations can send data in unused data-slots according to a
round-robin sending scheme (best-effort traffic)
N mini-slots
reservations
for data-slots
e.g. N=6, k=2
N * k data-slots
other stations can use free data-slots
based on a round-robin scheme
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.12
MACA - collision avoidance
MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver
with a short RTS packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as soon as it is
ready to receive
Signaling packets contain

sender address
 receiver address
 packet size
Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.13
MACA examples
MACA avoids the problem of hidden terminals

A and C want to
send to B
 A sends RTS first
 C waits after receiving
CTS from B
RTS
CTS
A
CTS
B
C
MACA avoids the problem of exposed terminals

B wants to send to A, C
to another terminal
 now C does not have
to wait for it cannot
receive CTS from A
RTS
CTS
A
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
RTS
B
MC SS05
C
3.14
MACA variant: DFWMAC in IEEE802.11
sender
receiver
idle
idle
packet ready to send; RTS
RxBusy
ACK
time-out 
NAK;
RTS
wait for the
right to send
time-out;
RTS
data;
ACK
RTS;
CTS
time-out 
data;
NAK
CTS; data
wait for
data
wait for ACK
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
RTS; RxBusy
3.15
Polling mechanisms
If one terminal can be heard by all others, this “central” terminal
(a.k.a. base station) can poll all other terminals according to a
certain scheme

now all schemes known from fixed networks can be used (typical
mainframe - terminal scenario)
Example: Randomly Addressed Polling





base station signals readiness to all mobile terminals
terminals ready to send can now transmit a random number without
collision with the help of CDMA or FDMA (the random number can be
seen as dynamic address)
the base station now chooses one address for polling from the list of
all random numbers (collision if two terminals choose the same
address)
the base station acknowledges correct packets and continues polling
the next terminal
this cycle starts again after polling all terminals of the list
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.16
ISMA (Inhibit Sense Multiple Access)
Current state of the medium is signaled via a “busy tone”





the base station signals on the downlink (base station to terminals) if the
medium is free or not
terminals must not send if the medium is busy
terminals can access the medium as soon as the busy tone stops
the base station signals collisions and successful transmissions via the
busy tone and acknowledgements, respectively (media access is not
coordinated within this approach)
mechanism used, e.g.,
for CDPD
(USA, integrated
into AMPS)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.17
Access method CDMA
CDMA (Code Division Multiple Access)

all terminals send on the same frequency probably at the same time and
can use the whole bandwidth of the transmission channel
 each sender has a unique random number, the sender XORs the signal
with this random number
 the receiver can “tune” into this signal if it knows the pseudo random
number, tuning is done via a correlation function
Disadvantages:

higher complexity of a receiver (receiver cannot just listen into the
medium and start receiving if there is a signal)
 all signals should have the same strength at a receiver
Advantages:

all terminals can use the same frequency, no planning needed
 huge code space (e.g. 232) compared to frequency space
 interferences (e.g. white noise) is not coded
 forward error correction and encryption can be easily integrated
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.18
CDMA in theory
Sender A
sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)
 sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B
sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)
 sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space

interference neglected (noise etc.)
 As + Bs = (-2, 0, 0, -2, +2, 0)
Receiver wants to receive signal from sender A

apply key Ak bitwise (inner product)
Ae = (-2, 0, 0, -2, +2, 0)  Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6
 result greater than 0, therefore, original bit was „1“


receiving B

Be = (-2, 0, 0, -2, +2, 0)  Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.19
CDMA on signal level I
data A
1
0
Ad
1
key A
key
sequence A
data  key
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
1
1
1
0
1
0
1
1
1
0
0
0
1
0
0
0
1
1
0
0
As
signal A
Real systems use much longer keys resulting in a larger distance
between single code words in code space.
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Ak
MC SS05
3.20
CDMA on signal level II
As
signal A
data B
key B
key
sequence B
data  key
1
0
Bd
0
0
0
0
1
1 0
1
0
1 0
0
0
0
1 0
1
1
1
1
1
1
0
0 1
1
0
1 0
0
0
0
1 0
1
1
1
Bs
signal B
As + Bs
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
Bk
MC SS05
3.21
CDMA on signal level III
data A
1
0
1
1
0
1
As + B s
Ak
(As + Bs)
* Ak
integrator
output
comparator
output
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.22
Ad
CDMA on signal level IV
data B
1
0
0
1
0
0
As + B s
Bk
(As + Bs)
* Bk
integrator
output
comparator
output
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.23
Bd
CDMA on signal level V
As + B s
wrong
key K
(As + Bs)
*K
integrator
output
comparator
output
(0)
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
(0)
MC SS05
?
3.24
SAMA - Spread Aloha Multiple Access
Aloha has only a very low efficiency, CDMA needs complex receivers to
be able to receive different senders with individual codes at the same
time
Idea: use spread spectrum with only one single code (chipping sequence)
for spreading for all senders accessing according to aloha
collision
sender A
sender B
1
0
0
1
1
narrow
band
1
send for a
shorter period
with higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
t
Problem: find a chipping sequence with good characteristics
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.25
Comparison SDMA/TDMA/FDMA/CDMA
Approach
Idea
SDMA
segment space into
cells/sectors
Terminals
only one terminal can
be active in one
cell/one sector
Signal
separation
cell structure, directed
antennas
TDMA
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
all terminals are
active for short
periods of time on
the same frequency
synchronization in
the time domain
FDMA
segment the
frequency band into
disjoint sub-bands
CDMA
spread the spectrum
using orthogonal codes
every terminal has its all terminals can be active
own frequency,
at the same place at the
uninterrupted
same moment,
uninterrupted
filtering in the
code plus special
frequency domain
receivers
Advantages very simple, increases established, fully
simple, established,
robust
inflexible, antennas
Disadvantages typically fixed
inflexible,
frequencies are a
scarce resource
flexible, less frequency
planning needed, soft
handover
complex receivers, needs
more complicated power
control for senders
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
capacity per km²
Comment
only in combination
with TDMA, FDMA or
CDMA useful
digital, flexible
guard space
needed (multipath
propagation),
synchronization
difficult
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
MC SS05
3.26